Superconducting magnets are becoming increasingly popular for electrical machines due to the high current densities they can withstand and the correspondingly high flux densities they can produce, often in the order of a few Tesla.
Although the high levels of flux densities are beneficial for increasing the torque densities of machines, shielding must be carefully utilised to contain the magnetic fields, particularly when the machines are located in sensitive environments. Such an environment would be an aerospace environment.
There are two methods commonly employed for magnetic shielding. The first of these include the use of high magnetic permeable shields which provide a magnetically permeable path for the magnetic flux which retains the magnetic fields within the machine. The second are eddy current shields which are generally electrically conductive layers which surround the machine and have eddy currents induced in them by the magnetic field radiating from the machine. These eddy currents set up opposing magnetic fields which retain the radiating field within the machine.
High permeability shields are beneficial as they will work on quasi-static and alternating fields and are made from readily available materials, ferromagnetic materials can also be used to house the machine. Further, the magnetic field is, in effect, reflected back into the machine which this leads to an increase in magnetic loading and torque density of the machine.
A downside to using high permeability materials is that the radial thickness required in the case of rotating machines is related to the strength of the field which needs to be shielded and the flux density saturation limits imposed by the high permeability material used in the shield. This results in prohibitively large shields, particularly in superconducting machines.
Eddy current shields are preferential for superconducting machines in that they can be made from a relatively inexpensive conductive material such as copper. As a rule they tend to be lighter than the high permeability alternatives, although this is dependent on the operating frequency of the machine.
Eddy current shields are disadvantageous in that they are only effective on moving fields above a particular frequency and the circulating eddy currents result in Joule heating losses in the shields. For large currents, this heat needs to be removed.
Other options include superconducting shields, which operate without loss (outside of providing the required cooling). However, superconducting shields are prone to failure during transient events where the shields experience an unexpectedly high magnetic field and quench as a result.
Although temporary failure of the shield may be acceptable in some circumstances, it is not acceptable for aero applications. Further, due to the fact that aero electrical systems generally have low inertia and large switching loads, they are prone to transients. This makes the use of current superconductor shields inappropriate.
This invention seeks to provide an improved magnetic shield for a superconducting machine.